They have none of the special skills you’d find in tunnel, Gunn, varicap, Zener ... the anode tied to ground through R1 and the cathode tied to the Vcc power rail. The diode is in reverse ...

You know just what to do. The ground pin of a LM7805 connects to the cathode of a TL431 programmable Zener diode pulled from an old scrapped TV. The diode is referenced to a voltage divider ...